Recently, as the miniaturization and weight reduction of electronic products, electronic devices, communication devices, and the like are rapidly progressing and the need for electric vehicles has been greatly increased in relation to environmental problems, there is also a growing demand for performance improvements in secondary batteries used as power sources for these products. Among them, the lithium secondary battery has been attracting considerable attention as a high-performance battery because of its high energy density and high standard electrode potential.
The lithium-sulfur (Li—S) battery is a secondary battery using a sulfur-based material having an S—S bond (sulfur-sulfur bond) as a positive electrode active material and using lithium metal as a negative electrode active material. Sulfur, which is the main material of the positive electrode active material has advantages that it is very rich in resources, is not toxic, and has a low atomic weight. In addition, theoretical discharging capacity of the lithium-sulfur battery is 1675 mAh/g-sulfur, and its theoretical energy density is 2,600 Wh/kg. Since the energy density of the lithium-sulfur battery is much higher than the theoretical energy density of other battery systems currently under study (Ni-MH battery: 450 Wh/kg, Li—FeS battery: 480 Wh/kg, Li—MnO2 battery: 1,000 Wh/kg, Na—S battery: 800 Wh/kg), the lithium-sulfur battery is the most promising battery among the batteries developed so far.
During the discharging reaction of the lithium-sulfur battery, an oxidation reaction of lithium occurs at the negative electrode (Abode) and a reduction reaction of sulfur occurs at the positive electrode (Cathode). Sulfur before discharging has an annular S8 structure. During the reduction reaction (discharging), as the S—S bond is cut off, the oxidation number of S decreases, and during the oxidation reaction (charging), as the S—S bond is re-formed, electrical energy is stored and generated using an oxidation-reaction reaction in which the oxidation number of S increases. During this reaction, the sulfur is converted from the cyclic S8 structure to the linear structure of lithium polysulfide (Li2Sx, x=8, 6, 4, 2) by the reduction reaction and eventually, when the lithium polysulfide is completely reduced, lithium sulfide (Li2S) is finally produced. By the process of reducing to each lithium polysulfide, the discharging behavior of the lithium-sulfur battery is characterized by exhibiting a step-wise discharge voltage unlike lithium ion battery.
Among lithium polysulfides such as Li2S8, Li2S6, Li2S4 and Li2S2, lithium polysulfide (Li2Sx, usually x>4) which has particularly a high oxidation number of sulfur is easily dissolved in an electrolyte solution. The polysulfide (S82−, S62−) dissolved in the electrolyte solution diffuses far away from the positive electrode, where the lithium polysulfide is generated, by the concentration difference. Thus, the polysulfide eluted from the positive electrode is lost to the outside of the reaction zone of the positive electrode, making it impossible to perform the stepwise reduction to lithium sulfide (Li2S). That is, since the lithium polysulfide which is separated from the positive electrode and the negative electrode and exists in a dissolved state cannot participate in the charging and discharging reaction of the battery, the amount of sulfur material involved in the electrochemical reaction at the positive electrode is reduced and as a result, the lithium polysulfide is a major factor in reducing the charging capacity and energy of the lithium-sulfur battery.
Furthermore, in addition to being floated or deposited in the electrolyte solution, the polysulfide diffused into the negative electrode reacts directly with lithium and sticks to the surface of the negative electrode in the form of Li2S, thus causing the problem of corrosion of the lithium negative electrode.
In order to minimize the elution and diffusion of polysulfide, studies are underway to modify the morphology of the positive electrode composites which are composites formed by supporting sulfur particles on various carbon structures or metal oxides.